The present invention relates to a brake system including electronic anti-lock control (ABS) and electronically controlled brake force distribution (EBV) by means of electrically operated hydraulic valves, of which the inlet valves (i.e. the valves controlling the pressure fluid flow to the individual wheel brakes) are open in their inactive position. This brake system further includes control electronics to generate the valve control signals in response to wheel sensor signals representative of the rotational behavior of the individual wheels, and an error monitoring device which responds upon the occurrence of defects or malfunctions and disconnects or deactivates the control at least in part.
In up-to-date conventional brake systems including anti-lock control, the braking pressure is controlled by electrically or electromagnetically controllable inlet and outlet valves. The inlet valves are arranged in the pressure fluid conduit from the master cylinder to the wheel brakes. The pressure fluid conduits, in which the outlet valves are arranged, lead from the wheel brakes to a supply reservoir or a pressure fluid return pump. To ensure the uncontrolled brake function when the control system is deactivated or the current supply is disconnected, the inlet valves are provided as SO-valves (i.e. open in their de-energized condition), and the outlet valves are provided as SG-valves (i.e. closed in their de-energized condition). To reliably prevent dangerous overbraking of the rear wheels in any situation, even upon deactivation of the control, conventional pressure reducing valves or so-called brake force regulators are inserted into the pressure fluid conduits leading to the rear wheels in such slip-controlled systems.
Further, it is known to bypass these brake force regulators during an anti-lock control operation for introducing a particularly high amount of braking pressure into the rear-wheel brakes during a control operation in case of need. Upon failure of the anti-lock controller, the brake force regulators will operate to full effect.
Still further, it is known from German DE-A 33 23 402 to utilize the electrically operable hydraulic valves, which are required for an anti-lock control operation, also to control the brake force distribution to the front axle and rear axle which is necessary as a result of dynamic axle load shifts. In this publication, the inlet valves in the lines leading to the rear wheels take the form of SG-valves. These valves effect introduction of the pressure into the wheel brakes of the rear wheels in a decelerated fashion, so that the rearwheel slip maximally adopts a predetermined percentage of the slip which occurs on the front wheels. The pressure fluid conduit leading to the rear wheels is permanently closed in such a brake system upon deactivation of the electronic control.
Attempts have been made, for cost reasons, to use anti-lock brake systems in which additional brake force regulators are avoided. An electronic control or regulation of brake force distribution by means of anti-lock control valves, in particular inlet valves, is desirable. Upon malfunction or deactivation of the controller due to a defect, however, braking pressure introduction, even into the rear-wheel brakes, should be possible now as before. Thus, it is inappropriate that the valve in the conduit leading to the rear wheels is a SG-valve.